Anchoring of cobalt hydroxide catalysts on nanozeolite crystals for photocatalytic water oxidation

• First report of use of nanozeolites as photocatalysts for water oxidation.
• A novel procedure for anchoring metal hydroxide on nanozeolite crystals is reported.
• Use of nanozeolites significantly improves catalytic performance as compared to micron-sized zeolites.
• Catalytic activity is recoverable.

Synthesis of cobalt hydroxide on ∼40 nm zeolite crystallites by treatment of Co2+-exchanged zeolite (Co-NZ) with either sodium hydroxide (Co-NZ-OH) or tetramethyl ammonium hydroxide (TMA–NZ) is being reported. With the NaOH-treated sample, an additional hydrothermal treatment was carried out (HT-Co-NZ-OH). X-ray photoelectron spectroscopy in the Co2p satellite region indicate the presence of Co (II). Transmission electron microscopy of HT-Co-NZ-OH showed nanometer thick film-like deposits of cobalt compound on the surface of the nanocrystal, whereas for TMA-NZ, the deposits were spherical ∼3 nm particles on the surface of the nanozeolite. Using the Ru(bpy)32+ – persulfate system, the photocatalytic activity of these materials for water oxidation was tested by measuring dissolved oxygen. The initial rate for TMA-NZ was 3.1 × 10−2 mol O2/mmol Co.s, whereas for HT-Co-NZ-OH, it was 1.4 × 10−2 mol O2/mmol Co.s. However, with the TMA-NZ sample, the photocatalytic rate dropped by an order of magnitude with the recovered catalyst, while the rate was unchanged for HT-Co-NZ-OH. With TMA-NZ, the ∼3 nm cobalt species on the zeolite are mechanically unstable and fall off during the photocatalytic reaction, whereas the cobalt deposits on the HT-Co-NZ-OH are stable. The catalytic performance of HT-Co-NZ-OH is an order of magnitude higher than comparably prepared micron sized zeolites, due to the better utilization of the cobalt on the surface of the nanozeolite. Internal porosity of the zeolite is still accessible with HT-Co-NZ-OH for photochemical reaction with access to the catalyst on the surface.

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